Auger Electron Spectroscopy (AES) is one of the common and useful methods used for the analysis of grain boundary segregation of solutes. We have applied this method by using a newly designed field emission electron gun for the small area analysis of grain boundaries in a high purity Fe-S alloy. It is found that sulfur segregates to grain boundaries and precipitates as Fe-sulfide, and that some of the precipitate particles consist of sulfide (FeS) and oxide. This result suggests that the oxide acts as the nucleus of FeS precipitation.